Periodic magnetically focused beam tube

ABSTRACT

The periodic magnet structure for a periodically focused beam tube is formed by a single pair of homogeneous slabs of magnetizable material that are permanently magnetized with a pattern of magnetic poles of alternating polarity taken along the direction of the beam path. In a sheet beam tube of preferred geometry, a pair of ceramic slabs are disposed to straddle the sheet beam and the internal planar faces of the ceramic slabs have circuits printed thereon for: electrical connections to all elements, beam forming electrodes, microwave interaction structure, edge focusing electrodes, and the beam collector electrode structure. The permanently magnetized slabs are disposed external to the vacuum envelope, straddling the printed circuit ceramic slabs, for focusing the beam.

Scott [451 Jan. 22, 1974 PERIODIC MAGNETICALLY FOCUSED BEAM TUBE [75]Inventor: Allan W. Scott, Los Altos, Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Aug. 7, 1972 [21] Appl. No.: 278,408

[52] US. Cl 315/35, 315/36, 315/535, 335/210 [51] Int. Cl. H01j 25/34[58] Field of Search..... 315/35, 3.6, 5.35; 335/210, 335/302 [56]References Cited UNITED STATES PATENTS 3,705,327 12/1972 Scott 315/3.5

2,812,470 11/1957 Cook et a1. 315/3.5 X

3,670,196 6/1972 Smith 3l5/3.5

2,911,555 11/1959 Sensiper et a1. 315/35 3,504,222 3/1970 Fukushima315/3.5 X

3,231,780 1/1966 Feinstein 315/3.5 X

3,610,999 10/1971 Falce 315/35 Primary Examiner Rudolph V. RolinecAssistant Examiner-Saxfield Chatmon, Jr. Attorney, Agent, orFirm-Stanley Z. Cole [5 7 ABSTRACT The periodic magnet structure for aperiodically focused beam tube is formed by a single pair of homogeneousslabs of magnetizable material that are permanently magnetized with apattern of magnetic poles of alternating polarity taken along thedirection of the beam path. In a sheet beam tube of preferred geometry,a pair of ceramic slabs are disposed to straddle the sheet beam and theinternal planar faces of the ceramic slabs have circuits printed thereonfor: electrical connections to all elements, beam forming electrodes,microwave interaction structure, edge focusing electrodes, and the beamcollector electrode structure. The permanently magnetized slabs aredisposed external to the vacuum envelope, straddling the printed circuitceramic slabs, for focusing the beam.

19 Claims, 12 Drawing Figures PATENTEB JAN 22 I974 SHEET 1 or 3 B G FFIG 2B PATENIEUJAHZE? m SHEET 3 [IF 3 FIG.8

PERIODIC MAGNETICALLY FOCUSED BEAM TUBE GOVERNMENT CONTRACT Theinvention herein described was made in the course of or under a contractwith the department of the U. S. Army.

DESCRIPTION OF THE PRIOR ART The present invention relates to the fieldof periodic permanent magnet focused microwave beam tubes such astraveling wave tubes and klystrons.

Heretofore, it has been proposed to print a microwave interactionstructure on the inner faces of a pair of mutually opposed and spacedapart ceramic slabs disposed straddling a sheet-shaped electron beam.The beam was electrostatically focused by means of printed circuitelectrodes carried on the ceramic slab either on the same face with thatof the circuit or on an opposite face. Tubes of this character aredisclosed and claimed in U. S. Pat. Nos. 3,549,852 issued Dec. 22, 1970;3,448,384 issued June 3, 1969; and U. S. Pat. application Ser. No.149,191, filed June 2, 1971 now US. Pat. No. 3,705,327.

The disadvantages. of utilizing electrostatic focusing elements are thatarcing and secondary electron emission exist between the focusingelements which are at different voltages in the beam path, and the fullvoltage applied to the tube is not available for microwave interaction,since the effective interaction voltage is a value between the focusingvoltages.

It is also known from the prior art that tubular or sheet electron beamsmay be magnetically focused by means of a periodic magnetic focusingstructure having a pole pattern which is characterized by poles ofalternating polarity taken in the direction of the beam path. In theseprior art periodic magnetic focusing structures, two types of geometrieshave been employed; one of which employs magnetic poles of the samepolarity disposed in registration transversely across the beam and thesecond geometry wherein poles of opposite polarity are disposed intransverse registration across the beam path. The magnetic focusingstructure of the first type having poles of the same polarity disposedtransversely of the beam path is disclosed in U. S. Pat. No. 3,102,211(see FIG. 13) issued Aug. 27, 1963. The second type of geometry whereinthe poles are of opposite polarity taken transversely of the beam pathis disclosed and claimed in U. S. Pat. No. 3,013,173 issued Dec. l2,1961.

However, in these prior art periodic magnetic focus ing structures, themagnet structure has been relatively complicated because it requires aplurality of permanently magnetized magnets separated by magneticallypermeable material or spacers of a different magnetic property. Thus,the resultant structure is relatively complicated requiring a relativelylarge number of parts which must be assembled around the envelope of thetube to provide a composite periodic permanent magnet beam focusingstructure.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved periodic magneticallyfocusedbeam tube.

In one feature of the present invention, the periodic magnetic focusingstructure comprises a structure of generally homogeneous permanentlymagnetizable material magnetized in a pattern of periodic permanentpoles of alternating polarity taken in a direction along the beam path,whereby a relatively complicated periodic permanent pole geometry isobtained with an extremely simple magnetic structure. In another featureof the-present invention, a microwave beam tube includes a microwaveinteraction structure formed on a major face of a dielectric slab facingthe electron beam and a permanently magnetized pair of slabs ofgenerally homogeneous magnetic material forms the beam focusingstructure disposed external of the vacuum envelope of the tube adjacentthe outside wall of the dielectric slab for causing the magnetic fieldsto permeate the dielectric slab and microwave interaction structure tofocus the beam internally of the vacuum envelope of the tube.

In another feature of the present invention, a microwave interactionstructure, electrostatic beam edge focusing electrodes, electricalconnections, beam forming electrode structure, and beam collectorelectrode structure are all formed, as by printing, on the common faceof a dielectric slab facing a sheet-shaped electron beam. A pair ofslabs of homogeneous permanently magnetizable material is disposedoverlaying the dielectric slab externally of the vacuum envelope. Themagnet structure is permanently magnetized in a pattern of periodicpermanent poles of alternating polarity taken in a direction along thebeam path, whereby an extremely simplified tube structure is obtained.

In another feature of the present invention, a thermionic cathode isdisposed intermediate the ends of two printed microwave interactionstructures carried from the same face of a dielectric slab forprojecting electron beams in opposite directions over the printedcircuits to provide two microwave tubes within a common envelope.

In another feature of the present invention, plural microwave tubes areprovided within a single envelope by printing a plurality of interactioncircuits in side-byside relation on a common dielectric slab andprojecting a stream of electrons over the printed circuits to obtain-aplurality of microwave tubes within a common envelope.

In another feature of the present invention, the permanently magnetizedhomogeneous slab of magnet material is flexible such that after apattern of poles has been charged into the slab it is deformed, such asinto a cylinder, to match the contour of the envelope of the beam to befocused.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded schematicperspective view of a microwave tube incorporating features of thepresent invention,

FIG. 1A is an enlarged detail view of an alternative edge sealembodiment for a portion of the structure of FIG. 1 delineated by linelA-IA,

FIG. 2A is a perspective schematic view of a periodic magnetic focusingpole structure useful in the tube of FIG. 1,

FIG. 2B is a perspective plot of the axial magnetic field intensity forthe structure of FIG. 2A,

FIG. 3A is a view similar to that of-FIG. 2 depicting an alternativeembodiment of the magnetic focusing structure for the tube of FIG. 1, I

FIG. 3B is a perspective plot of the transverse magnetic field intensityfor the structure of FIG. 3A,

FIG. 4 is a schematic cross-sectional view of a sheet electron beamdepicting the space charge focusing forces,

FIG. Sis a plot of transverse and lateral electric defocusing fieldforces versus transverse extent of the electron beam of FIG. 4,

FIG. 6 is an enlarged cross-sectional view of a beam edge portion of thetube structure of FIG. 1 depicting the beam edge electrostatic focusingelectrode structrue and its edge focusing electric field,

FIG. 7 is a plot of collector current in milliamps versus voltage on theelectrostatic beam edge focusing electrode structure,-

FIG. 8 is a view similar to that of FIG. 1 depicting an alternative tubestructure of the present invention, and

7 FIG. 9 is a schematic plan view of a combination oscillator andamplifier printed circuit tube incorporating features of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a printed circuit microwave beam tube 11 incorporating featuresof the present invention. A tube 11 includes a pair of ceramic slabs l2and 13, as of alumina or beryllia ceramic, sealed together in agas-tight manner in spaced apart relation via the intermediary of asuitable gastight spacing structure such as spacing ring 14 as of metalor ceramic. In an alternative embodiment of FIG. 1A, the ceramic slabsl2 and 13 are sealed at their peripheries, as by brazing, to metallicsealing frames 15 and 16 which in turn are sealed together as by heliarcwelding at 20. The frames 15 and 16 replace spacer l4 and serve as ameans for sealing the vacuum envelope of the tube.

A thermionic cathode emitter 17 is mounted to one of the ceramic plates12 and 13, such as the lower plate 12, by soldering the heater leads ofthe thermionic cathode 17 to metalized leads 10 formed on the inner faceof the lower ceramic plate 12. The thermionic cathode assembly 17 ispreferably disposed at one end of the ceramic tube 11 in the spacebetween the spaced ceramic plates 12 and 13 respectively.

A microwave interaction structure 18, such as a meanderline, aninterdigital line, a double meanderline, or an array of distributedinteraction klystron cavities or the like is formed on each of theopposed inner faces of the ceramic plates 12 and 13, respectively, and abeam collector electrode plate 119 is similarly formed on both of thelower and upper ceramic plates 12 and 13.

A pair of beam edge electrostatic beam focusing electrodes 21 aredisposed at the side of each of the microwave interaction structures 18on both the opposed inner surfaces of the lower and upper ceramic plates12 and 13, respectively.

A pair of electron gun beam forming electrodes 20 are formed on theopposed inner surfaces of the lower and upper dielectric plates 12 and13 between the upstream end of the microwave circuit 18 and thethermionic cathode emitter E7. The focus electrodes 21 are disposed intransverse registration across the beam path for initially transverselyfocusing the electron beam into the region between the printed slow wavecircuits.

Electrical connections are made to each of the various electrodestructures within the envelope by means of electrically conductive pinspassing through the respective ceramic plates 12 and 13 and beingconnected to the respective electrodes by means of metalized pads andmetalized holes bonded to the pins. The dielectric plates and spacerstructures 12, I3 and 14 are all bonded together in a gas-tight mannerto define a gastight envelope for the tube ill. The envelope isevacuated via a conventional pump-out tubulation, not shown, to asuitable low pressure, as of 10 torr.

A pair of sheet-shaped magnets 23 and 24 are disposed externally of thevacuum envelope abutting the outside faces of the lower and upperdielectric ceramic slabs l2 and 13, respectively. The sheet-shapedmagnets 23 and 24 are made of a generally homogeneous permanentlymagnetizable magnetic material, such as ferromagnetic material, orpermanently magnetizable particles embedded in a suitable binder such assilicon rubber. Suitable magnet materials include barium ferrite Ba Fm-0 preferably oriented, and barium ferrite embedded in silicon rubber.Such materials preferably have a high coercive force, as of 5 2,000Oresteds.

The sheet-shaped magnets 23 and 24 are permanently magnetized in apattern generally indicated with respect to magnet 24, such patternhaving a plurality of laterally directed magnetic poles of alternatingpolarity taken in a direction along the beam path as shown schematicallyin FIGS. 2 and 3. The permanent magnet pattern is readily charged intothe slab-shaped magnet 24 by placing the magnetic sheet 24 over asuitable magnetizing fixture consisting of a generally comb-shapedmagnetically permeable material, as of soft iron. The teeth of the combhave a vane shape generally conforming to the shape of the laterallydirected poles to be charged into the magnetic sheet 24. Energizingcoils are wound about the base of the individual vane-shaped teeth ofthe combshaped charging fixture with the direction of current throughadjacent coils being in the opposite direction such as to generate, whenenergized with current, poles of alternating polarity taken in thedirection down the spine of the comb-shaped charging fixture.

The magnetic fixture 24, which is to be charged, can be disposed betweenthe mutually opposed teeth of two such comb-shaped charging fixtureswith the transversely aligned opposed teeth of the combs being ofopposite magnetic polarity such that the magnetic sheet 24 is chargeduniformly through the sheet with poles of alternating polarity.Alternatively, the sheet 24 may be disposed over a single charging comband charged. In this case, the magnetic sheet 24 is permanentlymagnetized with transverse directed poles opposite the ends of each ofthe vane-shaped teeth of the charging fixture and with longitudinalpolarization in the outer regions of the fringing field between adjacentpoles. An advantage to making the magnet structure 23 and 24 by chargingthe desired pole pattern is that, once a charging fixture has been madethe desired magnet structure can be duplicated exactly without assemblyand fabrication of individual magnets and spacers. Also, the intensityand period of the periodic magnet structure is readily varied down thelength of the magnet structure by varying the ampere turns around theindividual vane-shaped teeth of the charging fixture and by varying thespacing between adjacent teeth of the charging fixture.

The permanently magnetized sheet magnets 23 and 24, with theirrespective pole patterns, are disposed on opposite sides of the ceramicsheets 12 and 13 with their magnetic poles in transverse registration.in one type of magnetic focusing, as shown in FIG. 2, the opposed polesin the opposed sheets 23 and 24 have the same magnetic polarity toproduce periodic magnetic focusing of the sheet-shaped beam in themanner as disclosed in U. S. Pat. No. 3,102,211. As an alternative, thepole patterns in the opposed magnetic sheets 23 and 24 may be arrangedsuch that opposed type magnetic poles are disposed in transverseregistration as shown in FIG. 3. This type of magnetic deflectionfocusing is disclosed and claimed in U. S. Pat. No. 3,013,173.

In a typical example of a watt, L-band, CW microwave amplifier tube 11,the slow wave circuit 18 has a lateral width of approximately 0.80inches and an axial length of approximately 5 inches. The electron beamhas a lateral width of approximately 0.700 inches and a thickness ofapproximately 0.055 inches. Slow wave circuits 18 are formed as byprinted circuit techniques on the inner major faces of the opposedceramic sheets 12 and 13, each slab having a thickness of approximately0.100 inches and being spaced apart from the opposed slab byapproximately 0.120 inches. The electrostatic beam edge focusingelectrodes 21 are spaced by approximately 0.060 inches from the adjacentedge of the slow wave circuit 18 and each electrode 21 has a lateralwidth of approximately 0.050 inches. The sheet-shaped magnets 23 and 24each have a lateral width of approximately 1.00 inches and a thicknessof approximately 0.100 inches and extend for substantially the entireaxial length of the tube. In this case, the magnet structure had aperiod of 0.5 inch from one north pole to the succeeding north poletaken along the beam path. The periodic beam focusing field had a peaklongitudinal component of magnetic field intensity in the midplane ofthe beam of 200 gauss.

In the case where the sheet-shaped magnets 23 and 24 are magnetizedstraight through, such that the material between adjacent poles is notpermanently magnetized, a magnetically permeable member, such as a sheetof soft iron, not shown, is preferably disposed over the outer majorface of the sheet-shaped magnets 23 and 24 to serve as a return magneticflux path of high magnetic permeability.

Referring now to FIGS. 4-6 the effect of the beam edge electrostaticfocusing electrodes 21 is shown. In FIG. 4, the sheet-shaped electronbeam is shown at 27 with electrostatic space charge defocusing forces asindicated by the arrows radiating away from the beam 27. From thedirection of the arrows, it is seen that the defocusing forces aresubstantially transverse in the midlateral sections of the beam 27 but,near the beam edges, the defocusing forces become substantially lateral.

Referring now to FIG. 5 there is shown the relative amplitude of thetransverse and lateral spacecharged defocusing electric fields. From thecurve it is shown that the transverse defocusing electric field fallsoff as the lateral defocusing field increases.

Referring now to FIG. 6 there is shown the focusing electric field linesproduced by the beam edge electrostatic focusing electrodes 21. The beam27 will have a certain beam voltage, as of +900 volts relative to thecathode potential. Typically, the beam voltage is at ground potentialwhich is also the potential on the slow wave circuit 18 and the cathode17 is run at a negative potential. The difference between the slow wavecircuit potential and the cathode potential corresponds to the beamvoltage.

The beam edge focusing electrodes 21 are operated at a potentialnegative with respect to the beam potential and potential of the circuit18. The electrostatic force on the electrons is indicated by arrows 28and it is seen that these arrows, in the mid-transverse plane of thebeam at the edge of the beam, have a maximum amplitude tending to forcethe electrons back toward the mid-lateral plane of the beam 27.

It turns out that the potential on the edge focusing electrodes 21relative to the beam potential and the potential on the circuit 18 isnot critical. This is shown in FIG. 7 wherein collector currents inmilliamps is plotted versus voltage on the edge focus electrodes 21.From the curve it is seen that once a minimum voltage has beenestablished between the electrostatic focus electrode and the beamvoltage and circuit potential, as of 50 volts, that a further increaseof the voltage difference has very little effect on beam transmission.It is also seen that a relatively high beam transmission efficiency isobtained, as of 95 percent.

In a typical example of a traveling wave tube of the type shown in Flg.1, beam transmission of 92 percent was obtainable. With full RF driveand 50 percent collector depression the beam transmission fell only topercent. The instantaneous bandwidth of the tube was greater than 20percent centered at an L-band frequency of approximately 1.1 GHz whileproviding 15 watts of CW output power. With these results at L- band, itis quite feasible to obtain approximately 2 kilowatts pulse power in thefrequency range of 3.1 to 3.5 GHz. The manufacturing cost of the printedcircuit tube of FIG. 1 is approximately one-tenth of the cost of aconventional traveling wave tube to obtain the same output performance.

Referring now to FIG. 8, there is shown an alternative microwave tubeembodiment 30 of the present invention. In FIG. 8, the tube 30 issubstantially the same as that previously described with regard to FIG.1 with the exception that the longitudinal and lateral extent of theceramic plates 12 and 13 has been extended to accommodate a plurality ofparallel microwave interaction circuits l8 and formed on both of themutually opposed faces of the ceramic plates 12 and 13, respectively. Inaddition, the thermionic cathode emitter assembly 17 has been moved tothe center of the tube and the collector electrodes 19 are disposed atopposite ends of the ceramic slabs 12 and 13 such that the electronbeams are directed from both sides of the thermionic cathode emitter 17toward opposite ends of the individual microwave interaction circuits 18to the collector assemblies 19 at opposite ends of the tube. In thismanner, a single cathode emitter 17 can serve to provide an electronbeam for a plurality of individual tubes connected in parallel. In aparticular example, as depicted in Flg. 8, there are 10 tubes connectedin parallel. The microwave circuits and the connections for theindividual tubes are readily formed by printed circuit techniques suchthat the fabrication cost for the 10 tubes is substantially the same asit would be for one tube. The'RF outputs of the individual circuits 18are taken out through suitable RF output connectors 3]. arrayed atopposite ends of the composite tube 30. The individual RF outputs may beused individually or connected in parallel for increasing the poweroutput capability of the tube 30. The thermionic cathode 17 maycomprise, for example, a directly heated thoriated tungsten ribbon.

Referring now to FIG. 9, there is shown, in plan view, an alternativemicrowave tube embodiment 35 of the present invention. In tube 35 ofFIG. 9 the microwave circuit 18 includes two circuit portions, circuitportion 18 is a forward wave amplifier circuit, and circuit portion 18is an interdigital backward wave oscillator circuit. The output of thebackward wave oscillator circuit 18 is fed into the input of the forwardwave slow wave circuit via printed circuit line 33 disposed at theupstream end of the electron beam. The beam is emitted by a thermionicemitter 17 such that the sheet beam is common to both slow wave circuitsl8 and 18' and is collected by a common collector electrode 19.

mogeneous permanently magnetizable member disposed on one side of saidsheet-shaped beam and said Although, thus far in the above description,the magl nets 23 and 24 have been described as they would be employedfor focusing a sheet beam it is to be understood that the technique isalso applicable to focusing of solid and hollow cylindrical vbeams. Inthe case of a solid cylindrical beam, the magnet may comprise a hollowcylinder which is charged with a pattern of axially spaced ring-shapedpole regions of alternating magnetic polarity taken in the axialdirection along the beam path.

The hollow cylindrical magnet may be formed by charging the pattern intoa magnetic cylinder or by charging the pattern into a sheet or slab offlexible magnetic material, such as silicon rubber sheet or slab havinga homogeneous suspension of permanently magnetizable magnetic particlesembedded therein, and then bending the magnetized sheet into a cylinderto form the axially spaced pole pattern of ring-shaped poles ofalternating polarity.

In the case of a hollow cylindrical beam, two such concentricallydisposed cylindrical magnets may" be employed for focusing the annularbeam passable coaxially of and between the pair of cylindricalpermanently magnetized magnet structures.

What is claimed is:

1. In a magnetically focused beam tube, an evacuable envelope structure,means within said envelope for forming and projecting a beam of chargedparticles over a predetermined beam path, electrical circuit meansdisposed along the beam path in electromagnetic wave energy exchangingrelation with the beam of charged particles, periodic magnetic focusmeans disposed along the beam path for producing a periodic magneticfield within the beam path for focusing the beam along a predeterminedbeam path, said periodic magnetic focus means including, a generallyhomogeneous structure consisting of permanently magnetizable material,said magnetizable structure being permanently magnetized in a pattern ofperiodic permanent poles of alternating polarity taken in a directionalong the beam path.

2. The apparatus of claim 1 wherein said beam forming means forms asheet-shaped beam, and wherein said homogeneous structure includes, agenerally homagnetizable member being permanently magnetized in apattern having periodic permanent poles of alternating polarity taken ina direction along the beam path.

3. The apparatus of claim ll wherein the period of said pattern ofpermanent magnetic poles changes in a direction taken along the beampath.

4. The apparatus of claim 1 wherein said magnetizable structure has aneven surface facing said beam.

5. The apparatus of claim 2 wherein said magnetizable structurecomprises a slab of substantially homogeneous permanently magnetizablematerial having a planar face facing said beam.

6. The apparatus of claim 5 wherein said magnet slab is a slab offerrite magnet material.

7. The apparatus of claim 5 wherein said magnet slab is a slab ofpermanently magnetizable magnet particles embedded in a binder material.

8. The apparatus of claim 7 wherein said binder material is flexiblesuch that said slab is flexible.

9. The apparatus of claim 2 including, a dielectric slab having majorand minor faces with a major face disposed facing a major face of saidsheet-shaped beam, and wherein said magnetizable member includes a slabof substantially homogeneous permanently magnetizable material disposedoverlaying said dielectric slab, and said magnetic slab member havingmajor and minor faces, and a major face of said magnetic member beingdisposed facing a major face of said dielectric slab.

10. The apparatus of claim 9 wherein said permanently magnetizablestructure includes a pair of said magnetizable slabs disposed onopposite sides of said sheet-shaped beam with their respective majorfaces disposed facing a corresponding major face of said sheet-shapedbeam. I

11. The apparatus of claim 9 wherein said electrical circuit is amicrowave interaction structure formed on said major face of saiddielectric slab which faces the corresponding major face of saidsheet-shaped beam.

12. The apparatus of claim 11 including, electrostatic edge focusingelectrode means disposed adjacent the edge regions of said slow wavecircuit in electrical insulative relation thereto and extending alongthe opposte edges of said sheet-shaped beam for constraining lateralexpansion of said sheet-shaped beam.

13. The apparatus of claim 12 wherein said edge focusing electrode meansis disposed on said major face of said dielectric slab which faces saidmajor face of said beam.

14. The apparatus of claim 13 wherein the beam of charged particles is abeam of electrons, and wherein said beam forming and projecting meansincludes a thermionic cathode emitter means disposed at the upstream endof said beam path for generating the beam of electrons, current carryinga lead means for supplying heater current to said thermionic cathodeemitter, and wherein said dielectric slab has an even major face facingsaid corresponding major face of said electron beam, and wherein saidslow wave circuit, edge focusing electrode means, and said heater leadmeans are all formed directly on and lie substantially entirely on saideven major face of said dielectric slab which faces saidsheet-shapedelectron beam.

15. The apparatus of claim 14 wherein said even major face of saiddielectric slab is planar.

16. The apparatus of claim 15 including beam collector electrode meansdisposed on and lying substantially entirely on said planar major faceof said dielectric slab at the terminal end of said beam path.

17. The apparatus of claim 16 including, beam focus electrode structuredisposed on and lying substantially entirely on said planar major faceof said dielectirc slab at the upstream end of said beam pathintermediate said cathode emitter means and said slow wave circuitmeans.

18. The apparatus of claim 11 wherein said electrical circuit meansincludes a pair of elongated microwave interaction structure meansdisposed of said major face of said dielectirc slab in end-to-endrelation, and wherein said beam forming and projecting means includes athermionic cathode emitter disposed intermediate the adjacent ends ofsaid pair of microwave interaction structure means for projecting a pairof electron beams in opposite direction along said pair of microwaveinteraction structure means.

19. The apparatus of claim 11 wherein said electrical circuit meansincludes a plurality of elongated microwave interaction structure meansdisposed on and lying substantially entirely on said major face of saiddielectric slab in side-by-side relation.

1. In a magnetically focused beam tube, an evacuable envelope structure,means within said envelope for forming and projecting a beam of chargedparticles over a predetermined beam path, electrical circuit meansdisposed along the beam path in electromagnetic wave energy exchangingrelation with the beam of charged particles, periodic magnetic focusmeans disposed along the beam path for producing a periodic magneticfield within the beam path for focusing the beam along a predeterminedbeam path, said periodic magnetic focus means including, a generallyhomogeneous structure consisting of permanently magnetizable material,said magnetizable structure being permanently magnetized in a pattern ofperiodic permanent poles of alternating polarity taken in a directionalong the beam path.
 2. The apparatus of claim 1 wherein said beamforming means forms a sheet-shaped beam, and wherein said homogeneousstructure includes, a generally homogeneous permanently magnetizablemember disposed on one side of said sheet-shaped beam and saidmagnetizable member being permanently magnetized in a pattern havingperiodic permanent poles of alternating polarity taken in a directionalong the beam path.
 3. The apparatus of claim 1 wherein the period ofsaid pattern of permanent magnetic poles changes in a direction takenalong the beam path.
 4. The apparatus of claim 1 wherein saidmagnetizable structure has an even surface facing said beam.
 5. Theapparatus of claim 2 wherein said magnetizable structure comprises aslab of substantially homogeneous permanently magnetizable materialhaving a planar face facing said beam.
 6. The apparatus of claim 5wherein said magnet slab is a slab of ferrite magnet material.
 7. Theapparatus of claim 5 wherein said magnet slab is a slab of permanentlymagnetizable magnet particles embedded in a binder material.
 8. Theapparatus of claim 7 wherein said binder material is flexible such thatsaid slab is flexible.
 9. The apparatus of claim 2 including, adielectric slab having major and minor faces with a major face disposedfacing a major face of said sheet-shaped beam, and wherein saidmagnetizable member includes a slab of substantially homogeneouspermanently magnetizable material disposed overlaying said dielectricslab, and said magnetic slab member having major and minor faces, and amajor face of said magnetic member being disposed facing a major face ofsaid dielectric slab.
 10. The apparatus of claim 9 wherein saidpermanently magnetizable structure includes a pair of said magnetizableslabs disposed on opposite sides of said sheet-shaped beam with theirrespective major faces disposed facing a corresponding major face ofsaid sheet-shaped beam.
 11. The apparatus of claim 9 wherein saidelectrical circuit is a microwave interaction structure formed on saidmajor face of said dielectric slab which faces the corresponding majorface of said sheet-shaped beam.
 12. The apparatus of claim 11 including,electrostatic edge focusing electrode means disposed adjacent the edgeregions of said slow wave circuit in electrical insulative relationthereto and extending along the opposte edges of said sheet-shaped beamfor constraining lateral expansion of said sheet-shaped beam.
 13. Theapparatus of claim 12 wherein said edge focusing electrode means isdisposed on said major face of said dielectric slab which faces saidmajor face of said beam.
 14. The apparatus of claim 13 wherein the beamof charged particles is a beam of electrons, and wherein said beamforming and projecting means includes a thermionic cathode emitter meansdisposed at the upstream end of said beam path for generating the beamof electrons, current carrying a lead means for supplying heater currentto said thermionic cathode emitter, and wherein said dielectric slab hasan even major face facinG said corresponding major face of said electronbeam, and wherein said slow wave circuit, edge focusing electrode means,and said heater lead means are all formed directly on and liesubstantially entirely on said even major face of said dielectric slabwhich faces said sheet-shaped electron beam.
 15. The apparatus of claim14 wherein said even major face of said dielectric slab is planar. 16.The apparatus of claim 15 including beam collector electrode meansdisposed on and lying substantially entirely on said planar major faceof said dielectric slab at the terminal end of said beam path.
 17. Theapparatus of claim 16 including, beam focus electrode structure disposedon and lying substantially entirely on said planar major face of saiddielectirc slab at the upstream end of said beam path intermediate saidcathode emitter means and said slow wave circuit means.
 18. Theapparatus of claim 11 wherein said electrical circuit means includes apair of elongated microwave interaction structure means disposed of saidmajor face of said dielectirc slab in end-to-end relation, and whereinsaid beam forming and projecting means includes a thermionic cathodeemitter disposed intermediate the adjacent ends of said pair ofmicrowave interaction structure means for projecting a pair of electronbeams in opposite direction along said pair of microwave interactionstructure means.
 19. The apparatus of claim 11 wherein said electricalcircuit means includes a plurality of elongated microwave interactionstructure means disposed on and lying substantially entirely on saidmajor face of said dielectric slab in side-by-side relation.